Molybdenum reaction with alkynes

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

Molybdenum and tungsten carbyne (alkylidyne) complexes frequently undergo 2+2 cycloaddition reactions with alkynes to give the corresponding metallacyclobutadiene... 

Other late transition metals used in special cases of hydrosilation include cobalt, iron, ruthenium (vide infra for reactions with alkynes), osmium, chromium, molybdenum, tungsten and copper. Metallocenes (see Metallocene Complexes) of early transition metals and lanthanides have also been found to catalyze the hydrosilation of a number of unsaturated compounds including alkenes and esters (vide infra). 

Metathesis with alkynes is also quite useful in synthesis,particularly for internal alkynes although terminal alkynes are not good partners in this reaction.358 internal metathesis reactions with alkynes are known,including the conversion of 439 to 440 (in 73% yield) in FUrstner s synthesis of prostaglandin E2-1,15-lactone.360 Note the use of a molybdenum metathesis catalyst for this reaction. Diynes also react with alkynes in an intermolecular reaction to form aromatic rings. An example is the conversion of 441 to a 6 1 mixture of 442/443, in 82% yield.36la a similar, palladium-catalyzed cycloaromatization is also known.362 The metathesis disconnections are... 

Scheme 56 Reaction of niobium and molybdenum disulfido complexes with alkyne...

The bulky thiolate ligand 2,4,6-triisopropylbenzenethiolate (tipt) forms the stable complex [Mo(tipt)4], which gives monoadducts with alkynes, nitriles, CO and other small ligands and, unlike [Mo(SBu )4], the molybdenum does not appear to be reduced in these reactions.182... 

As stated in the Introduction, Templeton7 published a comprehensive review entitled Four Electron Alkyne Ligands in Molybdenum(II) and Tungsten(II) Complexes, including halocarbonyl complexes, which dealt with the literature up to 1987. Hence, this section of the review deals with the literature on halocarbonyl alkyne and alkene complexes published from 1987 to mid-1995. The vast majority of papers that have appeared during this period are concerned with the reactions of [MI2(CO)3(NCMe)2] (M = Mo, W) and their derivatives with alkynes, which begin this section of the review. 

PhS, p-tolS) initially yields the bis(nitrile) complexes [MI2(CO) (NCMe)2(i72-RC2R )]. which dimerize to yield the iodo-bridged complexes [ M(/t-I)I(CO)(NCMe)(T)2-RC2R ) 2].245 The reactions of the diphenylacet-ylene molybdenum complex with donor ligands and alkynes are shown in Scheme 6. 

Thus far, the reactions of the diiodo-complexes [MI2(CO)3(NCMe)2] with alkynes have been described. In this section the reactions of other seven-coordinate molybdenum(II) and tungsten(II) halocarbonyls with alkynes are described. 

The [S + C=C] route is illustrated by reaction of disulfur dichloride or sulfur dichloride with alkynes (Scheme 9), or by episulfidation of alkenes by sulfur atom donors in the presence of molybdenum oxocomplexes (Scheme 10) . 2-Phenylthiirane is the most efficient sulfur donor in the molybdenum-catalyzed episulfidation and the reaction proceeds stereoselectively as yy/z-addition e.g., the episulfidation reaction of trans-cyclooctene affords /r my-epithiocyclooctane (Scheme 10) <2003JA3871>. 

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